Archived Kit Documentation

Unfortunately neither of the DCD kits are available anymore. The
bare board for the DCD State Machine is available.
Please contact us for details.

The TAPR 'DCD MODS'

BackgroundProper operation of Data Carrier Detect
(DCD) is imperative for efficient sharing of a packet
channel. Many TNC's don't provide optimum DCD operation,
and version 2.0 of the AX.25 Level 2 protocol compounds
the problem. However, an inexpensive solution is now
available to combat the former case and progress is being
made in the latter case with proposed changes to AX.25
Level 2 Version 2.1.

The Problem
Over time, it has become apparent that
most modems lack the proper DCD operation. Some work
well, some are worse than others, some allow improper
operator adjustment, and some have the 'threshold'
adjustment set incorrectly. With the release of TNC-2
code 1.1.6 and later, packets that are received without
the DCD being active are not passed to the user. This
encourages the operator to properly set any DCD threshold
control that may be on his TNC. Eric Gustafson, N7CL, has
investigated this problem and his findings are available
in the
7th ARRL Computer Networking Conference
proceedings "Can We Continue to Ignore Level One? ".
It has also been published in TAPR's
Packet Status Register.

Solution
If the DCD decision could be made on the
basis of 'information coherence' rather than 'is there
some sort of signal or noise present?', LAN operation
would improve. This premise has been demonstrated in a
number of locations where modifications to TNCs have been
made. Unfortunately, these early modifications involved
wire-wrapping a fair bit of circuitry, thus preventing
widespread adoption of the DCD mods. The TAPR DCD kits
were designed to make it a trivial matter to upgrade most
TNCs to improved DCD operation with the addition of a
simple board. These mods are extremely useful for both
VHF and HF operation.

If your TNC is not on this list, see the XR2211 DCD Modification (below),
which is designed for XR2211 based modems.

The upgrade adaptor for these TNCs adds an EPROM-based
State Machine to derive DCD based on lockup of a digital
phase-lock loop. It is a small PC board assembly that
mounts easily inside the cabinet of most any TNC (NOT the
Heath Pocket Packet/TASCO TNC-521).

This upgrade will dramatically improve DCD operation,
allowing you to run your radio unsquelched. This alone
reduces other stations' TXDelay requirements, improving
channel throughput.

Sorry, Clock Option (for DCD or 9600b modem) is no longer available

XR-2211 DCD Upgrade

If your TNC is not on this list, see the DCD State Machine
Modification (above), which is designed for non-XR2211 based modems.

The PC board is tiny and shaped to fit into a
TNC-1 or TNC-2. After construction (an hour or so),
you simply unplug the XR-2211 chip from its socket, insert
it into the socket on the upgrade board, then plug the
upgrade board into the IC socket vacated by the XR-2211
chip on the TNC.

This upgrade will dramatically improve DCD operation,
allowing you to run your radio unsquelched. This alone
reduces other stations' TXDelay requirements, improving
channel throughput. The result will be fast-attack, slow
decay DCD with a hang time to compensate for temporary fades
due to multipath. When all stations sharing a channel have
proper DCD action, data flows more efficiently.

If you are into HF operation, provisions are made to
connect a 'Threshold' control onto the demodulator.